The present invention relates to a device for measuring the bath temperature in a fused salt electrolytic cell used to produce aluminum, in particular in such a cell employing carbon anodes.
Large scale production of aluminum today is still mainly by fused salt electrolytic reduction of aluminum oxide, for which purposes the aluminum oxide is dissolved in a fluoride melt comprised for the greater part of cryolite. The cathodically precipitated aluminum collects on the carbon floor of the cell under the fluoride melt, the surface of this liquid aluminum forming the actual cathode. Dipping into the melt from above are anodes which in the conventional process are made of amorphous carbon. As a result of the electrolytic decomposition of the aluminum oxide, oxygen is formed at the carbon anodes and reacts with the carbon there to form CO.sub.2 and CO. The electrolytic process takes place in a temperature range of about 940.degree.-970.degree. C.
During the course of the electrolytic process, the electrolyte becomes depleted in aluminum oxide. At a lower concentration of 1 to 2 wt.% of aluminum oxide in the melt the anode effect takes place in that the voltage increases from from e.g. 4-5 V to 30 V and more. Then at the latest the aluminum oxide concentration of the bath must be increased by the addition of further alumina.
The present day, widely used electronic data processing methods used to regulate the cells require that the bath temperature be measured continuously. Any deviations from the desired temperature must be registered immediately. For this purpose direct methods of temperature measurement are employed e.g. by means of a thermocouple immersed in the bath, or via indirect methods such as an infra-red pyrometer.
Revealed in the German Pat. No. 28 44 417 are two thermocouple wires which are welded together and protected from attack by the molten electrolyte. These wires are embedded in an electrically insulating filler material enclosed in a protective tube, which is closed-off at one end, and is made up of multiple layers of metals and insulating materials. The protective tube is enclosed in a thick-walled graphite crucible. The upper part of this graphite crucible is protected by steel pipe which in turn is mainly covered over by a crust of solidified electrolyte.
Although this measuring facility has been able to fulfil the requirements of the aluminum producer in terms of mechanical properties and corrosion resistance in the chemically aggressive melt, it still exhibits certain disadvantages:
A considerable amount of heat is conducted via the protective pipe into the relatively cool region above the level of the bath, and some of this is lost via radiation and/or conduction. Consequently the temperature registered is too low.
The high heat capacity of the thick graphite wall leads to delays of about half an hour in indicating fluctuations in the bath temperature.
The thermocouple occupies considerable space and can therefore be positioned only beside the anode e.g. in the central gap at the longitudinal axis of the cell. This means that the temperature under the anode cannot be measured, which again leads to too low temperatures being recorded, and even these with a time delay.
Because of the crusting over of the thermocouples damage can occur to them when the anodes are changed.
The thermocouple according to the German Pat. No. 28 44 417 is relatively expensive.
The applicant, following a disclosed process, drilled an approximately vertical hole in a prebaked carbon anode, and mounted an infra-red pyrometer above this hole in line with its central axis above the reduction cell. In order to prevent the molten electrolyte from rising and solidifying in the hole, compressed air was introduced into the hole. The unfavorable location of the relatively expensive infra-red pyrometer in the anodic part of the cell, and the necessary injection of compressed air, together with the experience that in practice a crust-free and carbon-foam-free surface cannot be achieved, showed that this method of temperature measurement is not practical on an industrial scale.